Q: Why are dinosaurs extinct?
A: Because they didn’t have a space program.
66 million years ago an asteroid that was at least six miles wide smashed into what is now the Yucatan Peninsula. Releasing the energy of five billion Nagasaki-sized bombs, ejecting dust and soot from widespread fires into the atmosphere, blocking the sun, preventing photosynthesis, and dropping temperatures to freezing for at least a decade, maybe longer, it was a global catastrophe. It wiped out not only the dinosaurs, but 75% of all animal and plant species on Earth at that time.
https://upload.wikimedia.org/wikipedia/commons/thumb/8/8c/Chicxulub_impact_-_artist_impression.jpg/1024px-Chicxulub_impact_-_artist_impression.jpg
Could such a killer asteroid head our way again? It’s not a matter of if, but when.
How could we protect ourselves? We can’t just nuke it and expect it to disappear. That would more likely turn what is effectively a large caliber bullet into a 12-guage shotgun blast. Both are deadly. The best bet is to deflect it so that it misses the Earth altogether. The sooner we do that, the farther from the Earth we give it a nudge, the less force we will need to exert.
https://i.stack.imgur.com/xWgGB.png
The asteroid Didymos and its moon Dimorphos pose no threat to Earth. (Nor does any other large asteroid of which we are aware.) They do however offer the opportunity for a technical demonstration of our ability to change the path of an asteroid by smashing an object into it. You have probably seen this video of the final moments before the Double Asteroid Redirection Test (DART) spacecraft impacted on the moon of the larger asteroid.
The moon and the spacecraft were traveling toward each other at 15,000 miles per hour; this was a head-on collision. The spacecraft was “pushing back” against the moon’s direction of travel to slow it down. In addition, there was a plume of material ejected from the impact that acted the same way a jet exhaust would.
https://spaceweather.com/images2022/27sep22/dart_strip_opt.gif
The effect would be to slow the orbital speed of Dimorphos, drop it into a lower orbit where it would pick up speed, and thereby shorten its orbital period. Before the impact, Dimorphos orbited Didymos every 11 hours and 55 minutes. The impact shortened that period by 32 minutes.
https://upload.wikimedia.org/wikipedia/commons/thumb/5/52/Infographic_showing_the_effect_of_DART%27s_impact_on_the_orbit_of_Didymos_B.jpg/1920px-Infographic_showing_the_effect_of_DART%27s_impact_on_the_orbit_of_Didymos_B.jpg
This was a resounding success! One of the unknowns was the nature of the target. Was it a solid hunk of rock, or was it a “rubble pile” loosely held together by gravity? These would be affected differently by a kinetic impact, and certainly by something more energetic like a nuclear device. The final pictures before impact make it pretty clear that Dimorphos is a rubble pile!
https://upload.wikimedia.org/wikipedia/commons/thumb/8/84/Dimorphos_from_DART_aprox._3_sec_before_impact.jpg/800px-Dimorphos_from_DART_aprox._3_sec_before_impact.jpg
Analysis of all the data from the mission will be helpful in designing our planetary defense strategy. The other part of that strategy is finding dangerous objects while they are years away from crossing our path. But that is a post for another day.
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